It is often desirable to transfer a force between an actuator and a recipient in a mechanical system. In particular, it is often desirable to exert the force on a “virtual point” which may not be accessible in reality.
For example in the field of rehabilitation robots for victims of neurological disorders, the actuator needs to be able to provide both actuating and supporting forces to move and support patient limbs. This may be done, for example, by using a rod attached at one end to an actuator, and at the other end to the limb of a subject, such as an arm or leg.
The connection between the rod and the subject's limb may be achieved in several ways. For example, the rod may be rigidly attached to a strap or brace which secures around the limb. This type of connection does not allow any free movement between the rod and limb, and as such movement of the limb in all six degrees of freedom is dependent upon movement of the rod.
This is problematic in rehabilitation. The aim is to progress the subject towards self supporting motion, and allowing some movement is beneficial as the feeling of freedom of movement inspires the subject to exercise control.
A spherical or Cardan joint may be provided between the rod and strap, however this only provides freedom in two rotational degrees of freedom, about the centre of the physical joint. In the event that the force is applied to a joint, for example an ankle, this can be problematic, as once the ankle joint rotates about the spherical joint, the line of direction of force from the rod is no longer coincident with the centre of rotation of the ankle. This is not ideal, as it creates a moment on the ankle which resists movement back to the nominal, aligned position and therefore resists the subject's efforts to correct their gait.
What is required is a connection which allows transmission of a force from an actuator to a recipient, but minimises constraint free rotation of the recipient.